1. Field of the Invention
This invention relates to a light modulation medium having a light modulation element formed by laying plural light modulation layers to realize a multilayer structure and adapted to change the optical characteristic of each of the light modulation layers by controlling the electric field applied to the light modulation element and also to a light modulation method to be used with such a light modulation medium.
2. Description of the Related Art
Consumption of large volumes of paper has been a serious problem of modern societies because it leads to destructions of forest resources that supply raw materials of paper pulps and environmental pollutions due to dumping and burning of paper wastes. However, due to the development of information society that involves dissemination of personal computers and a wide use of the Internet, paper is being consumed at an increasing rate particularly as short-life documents that are used to allow temporary accesses to electronic data. Therefore, there is a strong demand for writable display mediums that can replace paper.
A display/memory medium that can store data without any power source, display a full color image and redraw it by means of an external device has been proposed along with a method and a device for drawing an image on such as display/memory medium (see Patent Document 1).
According to the prior art invention, a display/memory medium is prepared by arranging plural display layers between a pair of substrates to form a multilayer structure. The layers selectively reflect light of different colors and are made of respective cholesteric liquid crystals that show different threshold voltages relative to a write signal applied externally. The image drawing device is prepared separately from the display/memory medium and provided with a pair of write electrodes for holding the display/memory medium between them and a drive circuit. An image is displayed by applying a write signal that is a voltage selected from plural different voltages separated from each other by the threshold voltages of the cholesteric liquid crystals of the different display layers and has a refresh period, a select period and a subsequent display period when no voltage is applied and the voltage Vr that is applied in the refresh period and the voltage Vs that is applied in the select period show a relationship of Vr>Vs.
[Patent Document 1]
Japanese Patent Application Laid-Open Publication No. 2001-154219 (Paragraph No. 0093 through Paragraph No. 0107, FIGS. 6 through 12)
However, the light modulation layers of the display/memory medium need to have a wide margin of operation relative to each other in order to make them display an image with an enhanced level of contrast by means of the method disclosed in the above Patent Document 1.
In the case of a light modulation element having two light modulation layers laid one on the other to which a voltage is applied, the margin of operation Vm of the light modulation layers relative to each other is determined by the formula below;2×(Vpf90 of B−Vpf10 of A)/(Vpf90 of B+Vpf10 of A),where Vpf90 is the voltage at which the normalization reflectivity is 90% and Vpf10 is the voltage at which the normalization reflectivity is 10% when each of the light modulation layers moves from a planar state to a focal conic state, while B represents the light modulation layer that shows a higher threshold voltage and A represents a light modulation layer that shows a lower threshold voltage when moving from a planar state to a focal conic state.
Preferably, Vm has a positive value.
However, it is difficult to actually observe the margin of operation of each of the light modulation layers relative to each other in a state when a number of light modulation layers are arranged in a light modulation element. But, the margin of operation of each of the light modulation layers relative to each other can be confined within a predetermined range if the margin of operation can be substantially induced from the threshold electric field or the like of each of the light modulation layers as separated from each other before the layers are put together to form the light modulation element and each of the light modulation layers is designed to show the predetermined threshold value as used for the induction.
When each of the light modulation layers is formed from cholesteric liquid crystal showing positive dielectric constant anisotropy, the margin of operation for each of the light modulation layers to move from a planar state to a focal conic state is smaller than the margin of operation for it to move from a focal conic state to a homeotropic state so that the former margin of operation requires attention.